Internet Engineering Task Force (IETF) O. Troan, Ed.
Request for Comments: 7597 W. Dec
Category: Standards Track Cisco Systems
ISSN: 2070-1721 X. Li
C. Bao
Tsinghua University
S. Matsushima
SoftBank Telecom
T. Murakami
IP Infusion
T. Taylor, Ed.
Huawei Technologies
July 2015
Mapping of Address and Port with Encapsulation (MAP-E)
Abstract
This document describes a mechanism for transporting IPv4 packets
across an IPv6 network using IP encapsulation. It also describes a
generic mechanism for mapping between IPv6 addresses and IPv4
addresses as well as transport-layer ports.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7597.
Troan, et al. Standards Track [Page 1]

RFC 7597 MAP-E July 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Troan, et al. Standards Track [Page 2]

RFC 7597 MAP-E July 20151. Introduction
Mapping of IPv4 addresses in IPv6 addresses has been described in
numerous mechanisms dating back to the mid-1990s [RFC1933] [RFC4213].
The "automatic tunneling" mechanism as first described in [RFC1933]
assigned a globally unique IPv6 address to a host by combining the
host's IPv4 address with a well-known IPv6 prefix. Given an IPv6
packet with a destination address with an embedded IPv4 address, a
node could automatically tunnel this packet by extracting the IPv4
tunnel endpoint address from the IPv6 destination address.
There are numerous variations of this idea, as described in 6over4
[RFC2529], 6to4 [RFC3056], the Intra-Site Automatic Tunnel Addressing
Protocol (ISATAP) [RFC5214], and IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) [RFC5969].
The commonalities of all of these IPv6-over-IPv4 mechanisms are as
follows:
o Automatic provisioning of an IPv6 address for a host or an IPv6
prefix for a site.
o Algorithmic or implicit address resolution of tunnel endpoint
addresses. Given an IPv6 destination address, an IPv4 tunnel
endpoint address can be calculated.
o Embedding of an IPv4 address or part thereof into an IPv6 address.
In later phases of IPv4-to-IPv6 migration, it is expected that
IPv6-only networks will be common, while there will still be a need
for residual IPv4 deployment. This document describes a generic
mapping of IPv4 to IPv6 and a mechanism for encapsulating IPv4
over IPv6.
Just as for the IPv6-over-IPv4 mechanisms referred to above, the
residual IPv4-over-IPv6 mechanism must be capable of:
o Provisioning an IPv4 prefix, an IPv4 address, or a shared IPv4
address.
o Algorithmically mapping between an IPv4 prefix, an IPv4 address,
or a shared IPv4 address and an IPv6 address.
The mapping scheme described here supports encapsulation of IPv4
packets in IPv6 in both mesh and hub-and-spoke topologies, including
address mappings with full independence between IPv6 and IPv4
addresses.
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RFC 7597 MAP-E July 2015
This document describes the delivery of IPv4 unicast service across
an IPv6 infrastructure. IPv4 multicast is not considered in this
document.
The Address plus Port (A+P) architecture of sharing an IPv4 address
by distributing the port space is described in [RFC6346].
Specifically, Section 4 of [RFC6346] covers stateless mapping. The
corresponding stateful solution, Dual-Stack Lite (DS-Lite), is
described in [RFC6333]. The motivations for this work are described
in [Solutions-4v6].
[RFC7598] defines DHCPv6 options for the provisioning of MAP. Other
means of provisioning are possible. Deployment considerations are
described in [MAP-Deploy].
MAP relies on IPv6 and is designed to deliver dual-stack service
while allowing IPv4 to be phased out within the service provider's
(SP's) network. The phasing out of IPv4 within the SP network is
independent of whether the end user disables IPv4 service or not.
Further, "greenfield" IPv6-only networks may use MAP in order to
deliver IPv4 to sites via the IPv6 network.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Terminology
MAP domain: One or more MAP Customer Edge (CE) devices
and Border Relays (BRs) connected to the same
virtual link. A service provider may deploy
a single MAP domain or may utilize multiple
MAP domains.
MAP Rule: A set of parameters describing the mapping
between an IPv4 prefix, IPv4 address, or
shared IPv4 address and an IPv6 prefix or
address. Each domain uses a different
mapping rule set.
MAP node: A device that implements MAP.
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RFC 7597 MAP-E July 2015
MAP Border Relay (BR): A MAP-enabled router managed by the service
provider at the edge of a MAP domain. A BR
has at least an IPv6-enabled interface and an
IPv4 interface connected to the native IPv4
network. A MAP BR may also be referred to as
simply a "BR" within the context of MAP.
MAP Customer Edge (CE): A device functioning as a Customer Edge
router in a MAP deployment. A typical MAP CE
adopting MAP Rules will serve a residential
site with one WAN-side interface and one or
more LAN-side interfaces. A MAP CE may also
be referred to as simply a "CE" within the
context of MAP.
Port set: Each node has a separate part of the
transport-layer port space; this is denoted
as a port set.
Port Set ID (PSID): Algorithmically identifies a set of ports
exclusively assigned to a CE.
Shared IPv4 address: An IPv4 address that is shared among multiple
CEs. Only ports that belong to the assigned
port set can be used for communication. Also
known as a port-restricted IPv4 address.
End-user IPv6 prefix: The IPv6 prefix assigned to an End-user CE by
means other than MAP itself, e.g.,
provisioned using DHCPv6 Prefix Delegation
(PD) [RFC3633], assigned via Stateless
Address Autoconfiguration (SLAAC) [RFC4862],
or configured manually. It is unique for
each CE.
MAP IPv6 address: The IPv6 address used to reach the MAP
function of a CE from other CEs and from BRs.
Rule IPv6 prefix: An IPv6 prefix assigned by a service provider
for a mapping rule.
Rule IPv4 prefix: An IPv4 prefix assigned by a service provider
for a mapping rule.
Troan, et al. Standards Track [Page 6]

RFC 7597 MAP-E July 2015
Embedded Address (EA) bits:
The IPv4 EA-bits in the IPv6 address identify
an IPv4 prefix/address (or part thereof) or a
shared IPv4 address (or part thereof) and a
Port Set Identifier.
4. Architecture
In accordance with the requirements stated above, the MAP mechanism
can operate with shared IPv4 addresses, full IPv4 addresses, or IPv4
prefixes. Operation with shared IPv4 addresses is described here,
and the differences for full IPv4 addresses and prefixes are
described below.
The MAP mechanism uses existing standard building blocks. The
existing Network Address and Port Translator (NAPT) [RFC2663] on the
CE is used with additional support for restricting transport-protocol
ports, ICMP identifiers, and fragment identifiers to the configured
port set. For packets outbound from the private IPv4 network, the CE
NAPT MUST translate transport identifiers (e.g., TCP and UDP port
numbers) so that they fall within the CE's assigned port range.
The NAPT MUST in turn be connected to a MAP-aware forwarding function
that does encapsulation/decapsulation of IPv4 packets in IPv6. MAP
supports the encapsulation mode specified in [RFC2473]. In addition,
MAP specifies an algorithm to do "address resolution" from an IPv4
address and port to an IPv6 address. This algorithmic mapping is
specified in Section 5.
The MAP architecture described here restricts the use of the shared
IPv4 address to only be used as the global address (outside) of the
NAPT running on the CE. A shared IPv4 address MUST NOT be used to
identify an interface. While it is theoretically possible to make
host stacks and applications port-aware, it would be a drastic change
to the IP model [RFC6250].
For full IPv4 addresses and IPv4 prefixes, the architecture just
described applies, with two differences: first, a full IPv4 address
or IPv4 prefix can be used as it is today, e.g., for identifying an
interface or as a DHCP pool, respectively. Second, the NAPT is not
required to restrict the ports used on outgoing packets.
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RFC 7597 MAP-E July 2015
Traffic outside of the domain (i.e., when the destination IPv4
address does not match (using longest matching prefix) any Rule IPv4
prefix in the Rules database) is forwarded to the BR.
There are two types of mapping rules:
1. Basic Mapping Rule (BMR) - mandatory. A CE can be provisioned
with multiple End-user IPv6 prefixes. There can only be one
Basic Mapping Rule per End-user IPv6 prefix. However, all CEs
having End-user IPv6 prefixes within (aggregated by) the same
Rule IPv6 prefix may share the same Basic Mapping Rule. In
combination with the End-user IPv6 prefix, the Basic Mapping Rule
is used to derive the IPv4 prefix, address, or shared address and
the PSID assigned to the CE.
2. Forwarding Mapping Rule (FMR) - optional; used for forwarding.
The Basic Mapping Rule may also be a Forwarding Mapping Rule.
Each Forwarding Mapping Rule will result in an entry in the rule
table for the Rule IPv4 prefix. Given a destination IPv4 address
and port within the MAP domain, a MAP node can use the matching
FMR to derive the End-user IPv6 address of the interface through
which that IPv4 destination address and port combination can be
reached. In hub-and-spoke mode, there are no FMRs.
Both mapping rules share the same parameters:
o Rule IPv6 prefix (including prefix length)
o Rule IPv4 prefix (including prefix length)
o Rule EA-bit length (in bits)
A MAP node finds its BMR by doing a longest match between the
End-user IPv6 prefix and the Rule IPv6 prefix in the Mapping Rules
table. The rule is then used for IPv4 prefix, address, or shared
address assignment.
A MAP IPv6 address is formed from the BMR Rule IPv6 prefix. This
address MUST be assigned to an interface of the MAP node and is used
to terminate all MAP traffic being sent or received to the node.
Port-restricted IPv4 routes are installed in the rule table for all
the Forwarding Mapping Rules, and a default route is installed to the
MAP BR (see Section 5.4).
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RFC 7597 MAP-E July 2015
Forwarding Mapping Rules are used to allow direct communication
between MAP CEs; this is known as "Mesh mode". In hub-and-spoke
mode, there are no Forwarding Mapping Rules; all traffic MUST be
forwarded directly to the BR.
While an FMR is optional in the sense that a MAP CE MAY be configured
with zero or more FMRs -- depending on the deployment -- all MAP CEs
MUST implement support for both rule types.
5.1. Port-Mapping Algorithm
The port-mapping algorithm is used in domains whose rules allow IPv4
address sharing.
The simplest way to represent a port range is using a notation
similar to Classless Inter-Domain Routing (CIDR) [RFC4632]. For
example, the first 256 ports are represented as port prefix 0.0/8 and
the last 256 ports as 255.0/8. In hexadecimal, these would be
0x0000/8 (PSID = 0) and 0xFF00/8 (PSID = 0xFF), respectively. Using
this technique but wishing to avoid allocating the system ports
[RFC6335] to the user, one would have to exclude the use of one or
more PSIDs (e.g., PSIDs 0 to 3 in the example just given).
When the PSID is embedded in the End-user IPv6 prefix, it is
desirable to minimize the restrictions of possible PSID values in
order to minimize dependencies between the End-user IPv6 prefix and
the assigned port set. This is achieved by using an infix
representation of the port value. Using such a representation, the
well-known ports are excluded by restrictions on the value of the
high-order bit field (A) rather than the PSID.
The infix algorithm allocates ports to a given CE as a series of
contiguous ranges spaced at regular intervals throughout the complete
range of possible port-set values.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-----------+-----------+-------+
Ports in | A | PSID | j |
the CE port set | > 0 | | |
+-----------+-----------+-------+
| a bits | k bits |m bits |
Figure 2: Structure of a Port-Restricted Port Field
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RFC 7597 MAP-E July 2015
a bits: The number of offset bits -- 6 by default, as this excludes
the system ports (0-1023). To guarantee non-overlapping
port sets, the offset 'a' MUST be the same for every MAP CE
sharing the same address.
A: Selects the range of the port number. For 'a' > 0, A MUST
be larger than 0. This ensures that the algorithm excludes
the system ports. For the default value of 'a' (6), the
system ports are excluded by requiring that A be greater
than 0. Smaller values of 'a' exclude a larger initial
range, e.g., 'a' = 4 will exclude ports 0-4095. The
interval between initial port numbers of successive
contiguous ranges assigned to the same user is 2^(16 - a).
k bits: The length in bits of the PSID field. To guarantee
non-overlapping port sets, the length 'k' MUST be the same
for every MAP CE sharing the same address. The sharing
ratio is 2^k. The number of ports assigned to the user is
2^(16 - k) - 2^m (excluded ports).
PSID: The Port Set Identifier (PSID). Different PSID values
guarantee non-overlapping port sets, thanks to the
restrictions on 'a' and 'k' stated above, because the PSID
always occupies the same bit positions in the port number.
m bits: The number of contiguous ports is given by 2^m.
j: Selects the specific port within a particular range
specified by the concatenation of A and the PSID.
5.2. Basic Mapping Rule (BMR)
The Basic Mapping Rule is mandatory and is used by the CE to
provision itself with an IPv4 prefix, IPv4 address, or shared IPv4
address. Recall from Section 5 that the BMR consists of the
following parameters:
o Rule IPv6 prefix (including prefix length)
o Rule IPv4 prefix (including prefix length)
o Rule EA-bit length (in bits)
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RFC 7597 MAP-E July 2015
Figure 3 shows the structure of the complete MAP IPv6 address as
specified in this document.
| n bits | o bits | s bits | 128-n-o-s bits |
+--------------------+-----------+---------+-----------------------+
| Rule IPv6 prefix | EA bits |subnet ID| interface ID |
+--------------------+-----------+---------+-----------------------+
|<--- End-user IPv6 prefix --->|
Figure 3: MAP IPv6 Address Format
The Rule IPv6 prefix is common among all CEs using the same Basic
Mapping Rule within the MAP domain. The EA bit field encodes the
CE-specific IPv4 address and port information. The EA bit field,
which is unique for a given Rule IPv6 prefix, can contain a full or
partial IPv4 address and, in the shared IPv4 address case, a PSID.
An EA bit field length of 0 signifies that all relevant MAP IPv4
addressing information is passed directly in the BMR and is not
derived from the EA bit field in the End-user IPv6 prefix.
The MAP IPv6 address is created by concatenating the End-user IPv6
prefix with the MAP subnet identifier (if the End-user IPv6 prefix is
shorter than 64 bits) and the interface identifier as specified in
Section 6.
The MAP subnet identifier is defined to be the first subnet (s bits
set to zero).
Define:
r = length of the IPv4 prefix given by the BMR;
o = length of the EA bit field as given by the BMR;
p = length of the IPv4 suffix contained in the EA bit field.
The length r MAY be zero, in which case the complete IPv4 address or
prefix is encoded in the EA bits. If only a part of the IPv4
address / prefix is encoded in the EA bits, the Rule IPv4 prefix is
provisioned to the CE by other means (e.g., a DHCPv6 option). To
create a complete IPv4 address (or prefix), the IPv4 address suffix
(p) from the EA bits is concatenated with the Rule IPv4 prefix
(r bits).
The offset of the EA bit field in the IPv6 address is equal to the
BMR Rule IPv6 prefix length. The length of the EA bit field (o) is
given by the BMR Rule EA-bit length and can be between 0 and 48. A
length of 48 means that the complete IPv4 address and port are
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RFC 7597 MAP-E July 2015
embedded in the End-user IPv6 prefix (a single port is assigned). A
length of 0 means that no part of the IPv4 address or port is
embedded in the address. The sum of the Rule IPv6 Prefix length and
the Rule EA-bit length MUST be less than or equal to the End-user
IPv6 prefix length.
If o + r < 32 (length of the IPv4 address in bits), then an IPv4
prefix is assigned. This case is shown in Figure 4.
| r bits | o bits = p bits |
+-------------+---------------------+
| Rule IPv4 | IPv4 address suffix |
+-------------+---------------------+
| < 32 bits |
Figure 4: IPv4 Prefix
If o + r is equal to 32, then a full IPv4 address is to be assigned.
The address is created by concatenating the Rule IPv4 prefix and the
EA-bits. This case is shown in Figure 5.
| r bits | o bits = p bits |
+-------------+---------------------+
| Rule IPv4 | IPv4 address suffix |
+-------------+---------------------+
| 32 bits |
Figure 5: Complete IPv4 Address
If o + r is > 32, then a shared IPv4 address is to be assigned. The
number of IPv4 address suffix bits (p) in the EA bits is given by
32 - r bits. The PSID bits are used to create a port set. The
length of the PSID bit field within the EA bits is q = o - p.
| r bits | p bits | | q bits |
+-------------+---------------------+ +------------+
| Rule IPv4 | IPv4 address suffix | |Port Set ID |
+-------------+---------------------+ +------------+
| 32 bits |
Figure 6: Shared IPv4 Address
The length of r MAY be 32, with no part of the IPv4 address embedded
in the EA bits. This results in a mapping with no dependence between
the IPv4 address and the IPv6 address. In addition, the length of o
MAY be zero (no EA bits embedded in the End-user IPv6 prefix),
meaning that the PSID is also provisioned using, for example, DHCP.
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RFC 7597 MAP-E July 2015
The BR forwards traffic received from the outside to CEs using the
normal MAP forwarding rules.
6. The IPv6 Interface Identifier
The interface identifier format of a MAP node is described below.
| 128-n-o-s bits |
| 16 bits| 32 bits | 16 bits|
+--------+----------------+--------+
| 0 | IPv4 address | PSID |
+--------+----------------+--------+
Figure 8: IPv6 Interface Identifier
In the case of an IPv4 prefix, the IPv4 address field is right-padded
with zeros up to 32 bits. The PSID field is left-padded with zeros
to create a 16-bit field. For an IPv4 prefix or a complete IPv4
address, the PSID field is zero.
If the End-user IPv6 prefix length is larger than 64, the most
significant parts of the interface identifier are overwritten by the
prefix.
7. MAP Configuration
For a given MAP domain, the BR and CE MUST be configured with the
following MAP elements. The configured values for these elements are
identical for all CEs and BRs within a given MAP domain.
o The Basic Mapping Rule and, optionally, the Forwarding Mapping
Rules, including the Rule IPv6 prefix, Rule IPv4 prefix, and
Length of EA bits.
o Hub-and-spoke mode or Mesh mode (if all traffic should be sent to
the BR, or if direct CE-to-CE traffic should be supported).
In addition, the MAP CE MUST be configured with the IPv6 address(es)
of the MAP BR (Section 5.4).
7.1. MAP CE
The MAP elements are set to values that are the same across all CEs
within a MAP domain. The values may be configured in a variety of
ways, including provisioning methods such as the Broadband Forum's
"TR-69" Residential Gateway management interface [TR069], an
XML-based object retrieved after IPv6 connectivity is established, or
manual configuration by an administrator. IPv6 DHCP options for MAP
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RFC 7597 MAP-E July 2015
configuration are defined in [RFC7598]. Other configuration and
management methods may use the formats described by these options for
consistency and convenience of implementation on CEs that support
multiple configuration methods.
The only remaining provisioning information the CE requires in order
to calculate the MAP IPv4 address and enable IPv4 connectivity is the
IPv6 prefix for the CE. The End-user IPv6 prefix is configured as
part of obtaining IPv6 Internet access.
The MAP provisioning parameters, and hence the IPv4 service itself,
are tied to the associated End-user IPv6 prefix lifetime; thus, the
MAP service is also tied to this in terms of authorization,
accounting, etc.
A single MAP CE MAY be connected to more than one MAP domain, just as
any router may have more than one IPv4-enabled service-provider-
facing interface and more than one set of associated addresses
assigned by DHCP. Each domain within which a given CE operates would
require its own set of MAP configuration elements and would generate
its own IPv4 address. Each MAP domain requires a distinct End-user
IPv6 prefix.
MAP DHCP options are specified in [RFC7598].
7.2. MAP BR
The MAP BR MUST be configured with corresponding mapping rules for
each MAP domain for which it is acting as a BR.
For increased reliability and load balancing, the BR IPv6 address MAY
be an anycast address shared across a given MAP domain. As MAP is
stateless, any BR may be used at any time. If the BR IPv6 address is
anycast, the relay MUST use this anycast IPv6 address as the source
address in packets relayed to CEs.
Since MAP uses provider address space, no specific routes need to be
advertised externally for MAP to operate in IPv6 or IPv4 BGP.
However, if anycast is used for the MAP IPv6 relays, the anycast
addresses must be advertised in the service provider's IGP.
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RFC 7597 MAP-E July 20158. Forwarding Considerations
Figure 1 depicts the overall MAP architecture with IPv4 users
connected to a routed IPv6 network.
MAP uses encapsulation mode as specified in [RFC2473].
For a shared IPv4 address, a MAP CE forwarding IPv4 packets from the
LAN performs NAT44 functions first and creates appropriate NAT44
bindings. The resulting IPv4 packets MUST contain the source IPv4
address and source transport identifiers specified by the MAP
provisioning parameters. The IPv4 packet is forwarded using the CE's
MAP forwarding function. The IPv6 source and destination addresses
MUST then be derived as per Section 5 of this document.
8.1. Receiving Rules
A MAP CE receiving an IPv6 packet to its MAP IPv6 address sends this
packet to the CE's MAP function, where it is decapsulated. The
resulting IPv4 packet is then forwarded to the CE's NAT44 function,
where it is handled according to the NAT's translation table.
A MAP BR receiving IPv6 packets selects a best matching MAP domain
rule (Rule IPv6 prefix) based on a longest address match of the
packet's IPv6 source address, as well as a match of the packet
destination address against the configured BR IPv6 address(es). The
selected MAP Rule allows the BR to determine the EA-bits from the
source IPv6 address.
To prevent spoofing of IPv4 addresses, any MAP node (CE and BR) MUST
perform the following validation upon reception of a packet. First,
the embedded IPv4 address or prefix, as well as the PSID (if any),
are extracted from the source IPv6 address using the matching MAP
Rule. These represent the range of what is acceptable as source IPv4
address and port. Second, the node extracts the source IPv4 address
and port from the IPv4 packet encapsulated inside the IPv6 packet.
If they are found to be outside the acceptable range, the packet MUST
be silently discarded and a counter incremented to indicate that a
potential spoofing attack may be underway. The source validation
checks just described are not done for packets whose source IPv6
address is that of the BR (BR IPv6 address).
By default, the CE router MUST drop packets received on the MAP
virtual interface (i.e., after decapsulation of IPv6) for IPv4
destinations not for its own IPv4 shared address, full IPv4 address,
or IPv4 prefix.
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RFC 7597 MAP-E July 20158.2. ICMP
ICMP messages should be supported in MAP domains. Hence, the NAT44
in the MAP CE MUST implement the behavior for ICMP messages
conforming to the best current practice documented in [RFC5508].
If a MAP CE receives an ICMP message having the ICMP Identifier field
in the ICMP header, the NAT44 in the MAP CE MUST rewrite this field
to a specific value assigned from the port set. BRs and other CEs
must handle this field in a way similar to the handling of a port
number in the TCP/UDP header upon receiving the ICMP message with the
ICMP Identifier field.
If a MAP node receives an ICMP error message without the ICMP
Identifier field for errors that are detected inside an IPv6 tunnel,
a node should relay the ICMP error message to the original source.
This behavior SHOULD be implemented in accordance with Section 8 of
[RFC2473].
8.3. Fragmentation and Path MTU Discovery
Due to the different sizes of the IPv4 and IPv6 headers, handling the
maximum packet size is relevant for the operation of any system
connecting the two address families. There are three mechanisms to
handle this issue: Path MTU Discovery (PMTUD), fragmentation, and
transport-layer negotiation such as the TCP Maximum Segment Size
(MSS) option [RFC879]. MAP uses all three mechanisms to deal with
different cases.
8.3.1. Fragmentation in the MAP Domain
Encapsulating an IPv4 packet to carry it across the MAP domain will
increase its size (typically by 40 bytes). It is strongly
recommended that the MTU in the MAP domain be well managed and that
the IPv6 MTU on the CE WAN-side interface be set so that no
fragmentation occurs within the boundary of the MAP domain.
For an IPv4 packet entering a MAP domain, fragmentation is performed
as described in Section 7.2 of [RFC2473].
The use of an anycast source address could lead to an ICMP error
message generated on the path being sent to a different BR.
Therefore, using a dynamically set tunnel MTU (Section 6.7 of
[RFC2473]) is subject to IPv6 Path MTU black holes. A MAP BR using
an anycast source address SHOULD NOT by default use Path MTU
Discovery across the MAP domain.
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RFC 7597 MAP-E July 2015
Multiple BRs using the same anycast source address could send
fragmented packets to the same CE at the same time. If the
fragmented packets from different BRs happen to use the same
fragment ID, incorrect reassembly might occur. See [RFC4459] for an
analysis of the problem; Section 3.4 of [RFC4459] suggests solving
the problem by fragmenting the inner packet.
8.3.2. Receiving IPv4 Fragments on the MAP Domain Borders
The forwarding of an IPv4 packet received from outside of the MAP
domain requires the IPv4 destination address and the
transport-protocol destination port. The transport-protocol
information is only available in the first fragment received. As
described in Section 5.3.3 of [RFC6346], a MAP node receiving an
IPv4 fragmented packet from outside has to reassemble the packet
before sending the packet onto the MAP link. If the first packet
received contains the transport-protocol information, it is possible
to optimize this behavior by using a cache and forwarding the
fragments unchanged. Implementers of MAP should be aware that there
are a number of well-known attacks against IP fragmentation; see
[RFC1858] and [RFC3128]. Implementers should also be aware of
additional issues with reassembling packets at high rates, as
described in [RFC4963].
8.3.3. Sending IPv4 Fragments to the Outside
If two IPv4 hosts behind two different MAP CEs with the same IPv4
address send fragments to an IPv4 destination host outside the
domain, those hosts may use the same IPv4 fragmentation identifier,
resulting in incorrect reassembly of the fragments at the destination
host. Given that the IPv4 fragmentation identifier is a 16-bit
field, it could be used similarly to port ranges. A MAP CE could
rewrite the IPv4 fragmentation identifier to be within its allocated
port set, if the resulting fragment identifier space was large enough
related to the rate at which fragments were sent. However, splitting
the identifier space in this fashion would increase the probability
of reassembly collisions for all connections through the Customer
Premises Equipment (CPE). See also [RFC6864].
9. NAT44 Considerations
The NAT44 implemented in the MAP CE SHOULD conform to the behavior
and best current practices documented in [RFC4787], [RFC5508], and
[RFC5382]. In MAP address-sharing mode (determined by the MAP
domain / rule configuration parameters), the operation of the NAT44
MUST be restricted to the available port numbers derived via the
Basic Mapping Rule.
Troan, et al. Standards Track [Page 19]

RFC 7597 MAP-E July 201510. Security Considerations
Spoofing attacks: With consistency checks between IPv4 and IPv6
sources that are performed on IPv4/IPv6 packets received by MAP
nodes, MAP does not introduce any new opportunity for spoofing
attacks that would not already exist in IPv6.
Denial-of-service attacks: In MAP domains where IPv4 addresses are
shared, the fact that IPv4 datagram reassembly may be necessary
introduces an opportunity for DoS attacks. This is inherent in
address sharing and is common with other address-sharing
approaches such as DS-Lite and NAT64/DNS64. The best protection
against such attacks is to accelerate IPv6 deployment so that
address sharing is used less and less where MAP is supported.
Routing loop attacks: Routing loop attacks may exist in some
"automatic tunneling" scenarios and are documented in [RFC6324].
They cannot exist with MAP because each BR checks that the IPv6
source address of a received IPv6 packet is a CE address based on
the Forwarding Mapping Rule.
Attacks facilitated by restricted port set: From hosts that are not
subject to ingress filtering [RFC2827], an attacker can inject
spoofed packets during ongoing transport connections [RFC4953]
[RFC5961] [RFC6056]. The attacks depend on guessing which ports
are currently used by target hosts. Using an unrestricted port
set is preferable, i.e., using native IPv6 connections that are
not subject to MAP port-range restrictions. To minimize these
types of attacks when using a restricted port set, the MAP CE's
NAT44 filtering behavior SHOULD be "Address-Dependent Filtering"
as described in Section 5 of [RFC4787]. Furthermore, the MAP CEs
SHOULD use a DNS transport proxy [RFC5625] function to handle DNS
traffic and source such traffic from IPv6 interfaces not assigned
to MAP.
[RFC6269] outlines general issues with IPv4 address sharing.
Troan, et al. Standards Track [Page 20]

RFC 7597 MAP-E July 2015Appendix B. A More Detailed Description of the Derivation of the
Port-Mapping Algorithm
This appendix describes how the port-mapping algorithm described in
Section 5.1 was derived. The algorithm is used in domains whose
rules allow IPv4 address sharing.
The basic requirement for a port-mapping algorithm is that the port
sets it assigns to different MAP CEs MUST be non-overlapping. A
number of other requirements guided the choice of the algorithm:
o In keeping with the general MAP algorithm, the port set MUST be
derivable from a Port Set identifier (PSID) that can be embedded
in the End-user IPv6 prefix.
o The mapping MUST be reversible such that, given the port number,
the PSID of the port set to which it belongs can be quickly
derived.
o The algorithm MUST allow a broad range of address-sharing ratios.
o It SHOULD be possible to exclude subsets of the complete port
numbering space from assignment. Most operators would exclude the
system ports (0-1023). A conservative operator might exclude all
but the transient ports (49152-65535).
o The effect of port exclusion on the possible values of the
End-user IPv6 prefix (i.e., due to restrictions on the PSID value)
SHOULD be minimized.
o For administrative simplicity, the algorithm SHOULD allocate the
same or almost the same number of ports to each CE sharing a given
IPv4 address.
The two extreme cases that an algorithm satisfying those conditions
might support are when (1) the port numbers are not contiguous for
each PSID but uniformly distributed across the allowed port range and
(2) the port numbers are contiguous in a single range for each PSID.
The port-mapping algorithm proposed here is called the Generalized
Modulus Algorithm (GMA) and supports both of these cases.
Troan, et al. Standards Track [Page 29]

RFC 7597 MAP-E July 2015
For a given IPv4 address-sharing ratio (R) and the maximum number of
contiguous ports (M) in a port set, the GMA is defined as follows:
a. The port numbers (P) corresponding to a given PSID are
generated by:
(1) ... P = (R * M) * i + M * PSID + j
where i and j are indices and the ranges of i, j, and the PSID
are discussed below.
b. For any given port number P, the PSID is calculated as:
(2) ... PSID = trunc((P modulo (R * M)) / M)
where trunc() is the operation of rounding down to the nearest
integer.
Formula (1) can be interpreted as follows. First, the available port
space is divided into blocks of size R * M. Each block is divided
into R individual ranges of length M. The index i in formula (1)
selects a block, PSID selects a range within that block, and the
index j selects a specific port value within the range. On the basis
of this interpretation:
o i ranges from ceil(N / (R * M)) to trunc(65536/(R * M)) - 1, where
ceil is the operation of rounding up to the nearest integer and N
is the number of ports (e.g., 1024) excluded from the lower end of
the range. That is, any block containing excluded values is
discarded at the lower end, and if the final block has fewer than
R * M values it is discarded. This ensures that the same number
of ports is assigned to every PSID.
o PSID ranges from 0 to R - 1.
o j ranges from 0 to M - 1.
Troan, et al. Standards Track [Page 30]

RFC 7597 MAP-E July 2015B.1. Bit Representation of the Algorithm
If R and M are powers of 2 (R = 2^k, M = 2^m), formula (1) translates
to a computationally convenient structure for any port number
represented as a 16-bit binary number. This structure is shown in
Figure 9.
0 8 15
+---------------+----------+------+-------------------+
| P |
----------------+-----------------+-------------------+
| i | PSID | j |
+---------------+----------+------+-------------------+
|<----a bits--->|<-----k bits---->|<------m bits----->|
Figure 9: Bit Representation of a Port Number
As shown in the figure, the index value i of formula (1) is given by
the first a = 16 - k - m bits of the port number. The PSID value is
given by the next k bits, and the index value j is given by the last
m bits.
Because the PSID is always in the same position in the port number
and always the same length, different PSID values are guaranteed to
generate different sets of port numbers. In the reverse direction,
the generating PSID can be extracted from any port number by a
bitmask operation.
Note that when M and R are powers of 2, 65536 divides evenly by
R * M. Hence, the final block is complete, and the upper bound on i
is exactly 65536/(R * M) - 1. The lower bound on i is still the
minimum required to ensure that the required set of ports is
excluded. No port numbers are wasted through the discarding of
blocks at the lower end if block size R * M is a factor of N, the
number of ports to be excluded.
As a final note, the number of blocks into which the range 0-65535 is
being divided in the above representation is given by 2^a. Hence,
the case where a = 0 can be interpreted as one where the complete
range has been divided into a single block, and individual port sets
are contained in contiguous ranges in that block. We cannot throw
away the whole block in that case, so port exclusion has to be
achieved by putting a lower bound equal to ceil(N / M) on the allowed
set of PSID values instead.
Troan, et al. Standards Track [Page 31]